It would be the muscle inserts (tendons & ligaments positions) and bone structure (length, leverage, joint,etc) which in turn will depend on genitics. Genetics can be a racial thing in a lot of cases. More or less muscle fiber would have nothing to do with that "riding pants" outer sweep look. Placing the heels on a block or BB plates will focus the stress on the quads, no matter what the foot spacing would be for any leg exercise. This would also include front squats. Heavy squating will bring the full potentional of the leg shape and development out to it's fullest. If your luckly enought to have that full sweep than you will probably get it from serious regular squating, BB Hacks, Legs presses, etc.. But you might try a little wider than normal foot spacing. A extra wide footing will bring in the hams in also, to a lesser degree. In any event, a lot of guy's can develop a large sweep of the butt, rather than of the legs, from flat footed squating. So feel free to select your forms of leg exercises with caution. Good Luck.